Conventionally, a Pb—Sn-based plated steel sheet excellent in corrosion resistance, processability, solderability (weldability) and the like has long been used as a material for automobile fuel tanks, but recent stringent environmental restrictions on the use of Pb has made it difficult for use in this system. A variety of steel sheets have been proposed as an alternative. Above all, an Sn—Zn plated steel sheet is excellent in corrosion resistance, processability and profitability, and its use is becoming widespread.
A surface-treated steel sheet for fuel tanks, obtained by applying a treatment of hexavalent chromium-containing chromate onto Zn—Ni alloy plating, is disclosed in Japanese Unexamined Patent Publication (Kokai) Nos. 58-45396 and 5-106058. Also, a material obtained through hot-dip Zn-galvanization and chromate treatment is disclosed in Japanese Unexamined Patent Publication (Kokai) Nos. 10-168581 and 11-217682.
The treatment with a hexavalent chromium-containing solution is excellent in view of corrosion resistance and profitability, but hexavalent chromium is an environmental load substance of which restriction on use is becoming more limited. In order to solve this problem, there have been proposed, for example, a method of reducing hexavalent chromium in Japanese Unexamined Patent Publication (Kokai) No. 2006-028547 and a method using an Si-based chemical without chromium in Japanese Unexamined Patent Publication (Kokai) No. 2001-32085. However, depending on the corrosion resistance evaluation under severe conditions or depending on the welding conditions, the objective performance cannot be satisfactorily achieved by chromium-free conventional techniques. Also, a method using a trivalent chromium with less environmental load has been proposed, for example, in WO02/20874, but as described later, the conventional technique is a treatment based on a Zn-galvanized steel sheet and even when this treatment is directly applied to an Sn-based plated steel sheet with a different surface state, paint adhesion is insufficient.
On the other hand, in the case of a material for automobile fuel tanks, a steel sheet subjected to Pb—Sn-based plating known as terne plating has been used, but regulations in Europe inhibit use of Pb, and a hot-dip Al-plated steel sheet or a hot-dip Sn—Zn plated steel sheet comes into use.
In conventional techniques, a method of obtaining a treatment solution with good liquid stability by combining a trivalent chromium and an organic acid is disclosed in Japanese Unexamined Patent Publication (Kokai) Nos. 10-81977, 10-81976, 10-176279, 10-212586, 11-256354, 2001-181855 and 2002-146550, but the main aim thereof is to reduce the amount of hexavalent chromium in the treatment solution and due to lack of sufficient studies on the ratio of hydroxyl group/carboxyl group in the organic acid molecule, which is described later, satisfactory performance is not necessarily obtained in view of paint adhesion/water resistance. Similarly, in Japanese Unexamined Patent Publication (Kokai) No. 2001-335958, an organic acid having an optimal hydroxyl group/carboxyl group ratio, which is described later, is not applied and since enhancement of paint adhesion has not been studied, paint adhesion is poor. It is disclosed that a silicate can be coated by a two-step treatment, but a water-soluble silicate is generally a salt with an alkali metal and unlike water-dispersible silica, there is no effect of enhancing paint adhesion.
Furthermore, the inventions described in Japanese Unexamined Patent Publication (Kokai) Nos. 2002-256447 and 2004-346360 have premise on water washing after coating and drying, and therefore, in addition to the reasons above, the amount of the component dissolved increases in the film components resulting in poor paint adhesion. In WO02/20874 and Japanese Unexamined Patent Publication (Kokai) No. 2002-226981, paint adhesion has not been studied. An organic material having a specific hydroxyl group is not contained and as regards the water-dispersible silica, the combination of spherical silica and chain silica is not studied, as a result, the alkali resistance and paint adhesion are inferior.